Going with the herd makes us feel safe, but herd behavior can go very wrong. It's not just crowds panicking and trampling. Enough of a crowd, and enough familiarity, will let smart people make exactly the wrong choice.
We don't herd together accidentally. In dangerous situations, we group together for safety. In uncertain situations we watch other people and do what they do. And in familiar situations, we move in an established pattern. If the herds don't get too big, or too frantic, this works pretty well. Large groups of people do guide us, they do protect us, and they do move through a given situation in an orderly manner.
When the energy of the crowd heats up, or when the herd is forced through choke points, things start to go badly. Studies show that people begin making "involuntary movements," as the crowd around them gets more chaotic. Waves move through the crowd - often from the front backwards - knocking people off their feet and crushing them. When individuals in a crowd lose control, people get killed.
Human Herds and Shopping
Loss of control isn't the only problem with herd behavior. Sometimes people don't have to be forced to give up control of themselves - put them in the wrong herd and they give it up. One well-studied aspect of herd behavior is the motion of people in stores. Put an American in a store, and we drift to the right. and circle around the periphery of the store. A Brit will go left. (Yes, it does have to do with driving on a certain side of the road.) People will do quite a lot to stay with the pattern, even struggling past objects blocking their way, to "graze" in a familiar pattern. Exploiting this behavior worked for stores, as arranging a store in a certain way increased the likelihood that people would stumble on tempting items and toss them into their shopping cart.
Then a new mode of retail came up. Not every store let you chuck your purchases in a cart and forget about them. Some stores require you to look around and make note of items you want before getting to a point of sale. When this happens, a herd of people entranced, unthinking, milling around a store isn't an advantage. Consumers are supposed to notice products and remember what they want. People are supposed to think where they are going, and remember what they want when they get there.
Fortunately, engineering this is easy enough. Forcing people to go directly against their milling instincts, making them go clockwise if they're inclined to go counterclockwise, helps them get a better "mental map" of the store and increased sales. Forcing people against their inclination, and the inclination of the group, makes them wake up.
Together, these two strategies allow retailers to control crowds. Put people in unfamiliar situations, where they have go against their "grazing" instincts, and they pay attention. Allow them to drift peacefully with a large group in a familiar pattern, and they become compliant. They go to sleep.
Human Herds and Danger
That's the problem with herd behavior. It puts us to sleep, and makes us stop assessing what's in front of us. In stores, there are no major consequences for this - other than coming home with jars of fancy olives that you'll never use. In more critical situations, this can be a real problem. Put people with a herd, allow the herd to drift along a familiar course, and people will literally walk to their death.
One study assessed the causes of "recreational avalanche accidents." Some of the results were pretty intuitive. People who had avalanche safety training were less likely to get into avalanche accidents. Then there were more disquieting results. At times, all that training counted for nothing. People would stop thinking about what they knew.
Going over unfamiliar ground carries risks, and so many accidents, at least with experienced trekkers, should happen in new or rarely-traveled areas. Actually, familiar areas were more likely to tumble down on people's heads, and not just because more people traveled on them. When we know where to go and how to move without thinking about it, we do exactly that. Familiar store layouts let herds of people drift through them without really thinking. Familiar trail layouts do the same. People who should know better turn off their judgment.
They also turn off their judgment in the presence of people - any other people. Avalanche training reduced the amount of risk to the person, but their risk of getting involved in accidents shot up when other people were around. When a group decides on a plan, they are unlikely to change it, keeping people walking along a route even when they should notice it's dangerous. Groups also make people feel safe, letting them take more dangerous courses. When people see others, or even past evidence of others, at a site, they keep to the established path even when they were trained to know better. If other people have done it, or are doing it, it has to be okay. Supposedly, putting a person with safety training with a group will let that person guide the group out of dangerous situations. Actually, it seems that putting trained people with a group will get the "leaders" to drift with the group.
And that's the problem with herd behavior. There are plenty of ways that people risk themselves, but most of them involve the people knowingly taking the risk. When herd behavior takes over, people don't realize when they are taking risks. They stop analyzing the situation, and go with the group, with the plan, with the familiar way. That makes it easy to drift into disaster without ever being aware that they're in danger.